1. Field of the Invention
The present invention relates generally to motor braking systems and more specifically to a plugging routine for accurately stopping a motor operated positioning device.
2. Brief Description of the Prior Art
Various devices have been proposed heretofore for the purpose of providing greater accuracy in stopping electrical motors operating in positioning devices. In many systems such as that disclosed in U.S. Pat. No. 3,694,671 wherein a fluid motor in earth moving equipment was considered, a single reverse polarity pulse was applied in response to termination of a signal operating the equipment in a selected direction. It should be appreciated that in such system, the duration of the reverse polarity pulse was fixed by a resistance-capacitance network; and therefore the timing could not adequately accommodate variations in equipment inertia. Thus, for example with the equipment moving a heavy load, there was a tendency for continued system movement even after the reverse pulse.
A further approach toward motor braking in a positioning device was illustrated in U.S. Pat. No. 3,995,208. In this apparatus a cyclic drive signal was applied to a motor and braking pulses were provided by shunting the motor terminals. The duration of the braking (shunting) pulses was controlled by a position responsive feedback network which controlled the operation of variable cycle timers. It should be appreciated that this prior system involved a significant degree of complexity because signals representative of the desired position and the present position were required to be differentiated and used for timer control. Thus, system cost was considerable and probably offset any advantage which might be obtained through the use of motors having relatively narrow tolerance limits in conjunction with a fixed duration shunt period.
Applicant had encountered, in the design of postage meter setting devices, substantial problems in providing an accurate yet economical motor braking system which could quickly position a meter setting lever.
Incorporation of feedback circuitry monitoring the exact position of the device and providing a corresponding braking signal dependent on such position involved additional costs and did not necessarily promote rapid braking cycles. On the other hand, the employment of a reverse polarity or shunting cycle fixed duration to a motor did not permit sufficient latitude for practical mass production motor tolerance limitations, thereby manadating high motor costs.
With practical motor tolerance limits it would have been necessary to determine the cycle time to halt the positioning device for each individual unit and employ a plugging cycle of the determined cycle time, which technique was highly impractical.